1. Field of the Invention
This invention relates to non-volatile memories including EPROM, EEPROM, and flash memory devices and more particularly to the form of programming voltages applied to non-volatile memory devices which utilize Fowler-Nordheim programming, hot electron injection or avalanche injection as the method of programming such devices.
2. Description of Related Art
Electrons in the floating gate of non-volatile memories repel electrons. By applying a programming pulse with a square shaped trace, the tunnelling current produces spikes at the start of a programming step and decreases dramatically afterwards. This results in long programming times. Such a voltage spike degrades the tunneling oxide layer. This results in a small number of programming cycles available before the tunneling oxide layer fails to be an insulating layer.
U.S. Pat. No. 5,485,423 of Tang for "Method for Eliminating of Cycling-Induced Electron Trapping in the Tunneling Oxide of 5 Volt Only EEPROMS" shows a method for applying a relative low positive voltage to a source region of a EEPROM during the erase cycle. However, this reference differs from the type of waveform voltage of the program and erase of the invention.
U.S. Pat. No. 5,042,009 of Kazerounian for "Method for Programming a Floating Gate Memory Device" shows a method of programming a floating gate using a charge pump to provide a drain current. The gate voltage rises from a first value to a second value during programming. However, the gate voltage is used for the limitation of the drain current rather than to produce a constant programming current for the optimization of programming speed and number of programming cycles.
See U.S. Pat. No. 4,434,478 of Cook et al. for a system and method for "Programming Floating Gate Devices". Cook et al. describes incrementing the gate voltage in a stepped or a continuously increasing voltage such as a linear or exponential ramp.